We propose to use confocal microscopy to examine the effect of immunotoxin treatment on HeLa spheroids. This involves two main sets of experiments: the first would measure the distribution of fluorescently-labeled immunotoxin in spheroids, and the second would examine immunotoxin-induced cytotoxicity as a function of position in the spheroid using a live/dead staining system and/or a probe for metabolic activity such as Rhodamine 123. The results of these experiments would then be used to confine or modify a mathematical model which describes immunotoxin penetration into spheroids and relates it to cytotoxicity.